Protective boot for a piston-cylinder unit of a disk brake

ABSTRACT

A flexible boot for protecting a cylinder-piston unit of a disk brake including a piston slidable in a cylinder in order to act on a brake pad, the flexible boot having a surface comprising a portion which can face a wall of the pad. The flexible boot comprises at least one spacer element which is associated with the surface and is intended to come into contact with the wall of the pad in order to keep the said portion of the surface spaced from the wall of the pad.

SCOPE OF THE INVENTION

[0001] The present invention relates to flexible boots, also known asdust guards for protecting piston-cylinder units usable in disk brakes.

BACKGROUND OF THE INVENTION

[0002] In general, in a disk brake, the braking force is produced bypiston-cylinder units formed in a suitable caliper and each comprising acylinder in which a piston can slide so as to be able to act, by meansof pads, on opposed surfaces of a rotating disk.

[0003] The piston-cylinder unit typically has a flexible boot forprotecting the cylinder and the piston (particularly their slidingsurfaces) against the entry of foreign material and/or corrosive agentssuch as, for example, dust, dirt, or moisture, which may prevent thepiston sliding correctly inside the cylinder during a braking operationand/or may give rise to corrosion of the sliding surfaces of the pistonand of the cylinder.

[0004] The flexible boots normally in use are bellows-shaped and such aboot is fitted on the piston-cylinder unit by fixing one of its endsaround the piston and an opposite end around the cylinder so as to forma protective barrier which extends around the piston-cylinder unit inthe region of the opening of the cylinder.

[0005] When the flexible protective boot is in the rest condition orwhen it is fitted in a disk brake in which the piston is disposedentirely inside the cylinder, as occurs with pads which are not worn,the boot adopts a configuration in which some of its portions are rolledup around one another so as to form one or more bends or pleats. Itshould be noted that, when the flexible boot is fitted on a disk brake,these pleats have convex portions facing towards the pad and, inparticular, towards a support plate of a friction lining of the pad. Asa result of wear of the friction linings of the pads, the piston extendstowards the disk and the flexible boot stretches out to adapt to the newposition adopted by the piston.

[0006] It is known that, during the assembly of the disk brake and, inparticular, during the installation of the piston-cylinder unit and itsflexible protective boot, a certain amount of air may remain trapped inthe space between the boot and the piston-cylinder unit.

[0007] During a braking operation, because of the overheating of thedisk on which the pad acts, heat is emitted and is transmitted to thetrapped air by the piston, increasing the pressure which the air exertson the flexible boot. This pressure may cause expansion of the flexibleboot such as to bring its pleats or other portions of the boot intodirect contact with the pad and, in particular, with the support plate.This contact is particularly likely in disk brakes having new, that is,unworn friction linings, in which the piston is substantially entirelyretracted into the cylinder and the convex portions of the pleats of theflexible boot are close to the support plate.

[0008] It is pointed out that, as a result of the braking operation and,in particular, if this operation is prolonged, the pad is at a hightemperature and may therefore burn or in any case damage the flexibleboot with which it is in contact, necessitating its replacement if theproblem is noticed.

[0009] Moreover, the problem of the contact between the flexible bootand the pad is particularly marked in disk brakes using floatingcalipers which require flexible boots of considerable extent so that theboots have several pleats that come close to the pad.

[0010] A method proposed by the prior art for preventing this damageprovides for the use of a flexible boot having a suitable hole for theoutlet of trapped air. According to this technique, the discharge of theair prevents bulging of the boot and hence contact with the pad.

[0011] As is known from U.S. Pat. No. 4,809,821, a further methodprovides for the formation, in the vicinity of the cylinder, of asuitable annular channel in which the pleats forming the flexible bootcan be housed completely so that they remain a suitable distance fromthe pad.

[0012] However, it has been found that the methods proposed up to now donot prevent contact between the flexible protective boot and the pad orthe consequent burning which damages the boot.

[0013] In fact, because of the high temperatures which may be reachedduring braking, the use of air-outlet holes does not suffice entirely toprevent the expansion of the air and of the boot trapping it.

[0014] It has also been noted that contact between the boot and the padis not only brought about by the above-mentioned expansion of the airbut may be due to incorrect positioning of the pleats which form theboot. For example, an incorrect arrangement of the pleats of the bootmay occur during assembly by the manufacturer or, more probably, as aresult of the refolding of the flexible boot following the replacementof the pads by a maintenance person. In fact, the replacement of thepads, which requires the piston to be returned into the cylinder, is anoperation which may be performed in conditions such as not always toensure correct refolding of the boot because of the limited space whichis left free by the pads that have been removed.

[0015] It should also be borne in mind that, as well as complicating andweakening the structure of the caliper in which the cylinder is formed,the presence of a suitable channel for completely housing the flexibleboot does not solve the problem created by incorrect mounting of theboot or by the bulging brought about by the heating of trapped air.

OBJECTS OF THE INVENTION

[0016] The problem upon which the present invention is based is that ofproposing a flexible boot for protecting a piston-cylinder unit usablein a disk brake which does not have the disadvantages indicated abovewith reference to the prior art.

SUMMARY OF THE INVENTION

[0017] This problem is solved by means of a flexible boot for protectinga piston-cylinder unit of a disk brake including a piston slidable in acylinder in order to act on a brake pad, the flexible boot having asurface comprising a portion which can face a wall of the pad,characterized in that it comprises at least one spacer element which isassociated with the surface and is intended to come into contact withthe wall of the pad in order to keep the said portion of the surfacespaced from the wall of the pad.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Further characteristics and the advantages of the invention willbecome clear from the following description of preferred embodimentsthereof, given by way of non-limiting example with reference to theappended drawings, in which:

[0019]FIG. 1 is partially-sectioned plan view of a disk brake accordingto the invention,

[0020]FIG. 2 is shows a portion of the disk brake of FIG. 1, on anenlarged scale,

[0021]FIG. 3 shows a portion of the disk brake of FIG. 1 with fully wornpads, on an enlarged scale,

[0022]FIG. 4 shows a first embodiment of a flexible boot according tothe invention,

[0023]FIG. 5 shows a second embodiment of a flexible boot according tothe invention, and

[0024]FIG. 6 shows a third embodiment of a flexible boot according tothe invention.

DETAILED DESCRIPTION OF THE INVENTION

[0025]FIGS. 1 and 2 show, in general, a disk brake 1 for acting on abraking band of a disk 2, particularly for a motor vehicle, not shown.

[0026] In the particular embodiment shown in FIG. 1, the disk brake 1 isof the floating-caliper type, but the present invention may be appliedto disk brakes of different types such as, for example, fixed-caliperdisk brakes.

[0027] The disk brake 1 comprises a disk 2 for association with a wheelof the motor vehicle, and a caliper 3 including a bracket 4 and afloating body 5 mechanically coupled to the bracket 4.

[0028] The bracket 4, which can be fixed to a stub axle of themotor-vehicle suspension, has a first bridge element 6 and a secondbridge element 7 which are substantially U-shaped and are arrangedastride the disk 2 on the left-hand side and on the right-hand side ofan axis I-I of the disk brake 1, respectively. The first and secondbridge elements 6 and 7 are connected to one another rigidly by means ofa first cross-member 8 and a second cross-member 8′ which extendsubstantially perpendicular to the axis I-I, on opposite sides of thedisk 2.

[0029] Moreover, two seats 11 and 12 are formed in the bracket 4 forhousing respective pads 13 and 14. The pads 13 and 14 compriserespective plates 25 and 26 for supporting corresponding frictionlinings 35 and 36.

[0030] The floating body 5 is arranged astride the disk 2 and the pads13 and 14 and comprises a side 40 which extends beside the plate 25 anda side 41 which extends beside the plate 26 and has a projection 42having a flat surface which can act on the plate 26.

[0031] The side 40 of the floating body 5 has, in the region of oppositeends of the axis I-I, respective cylindrical seats 43, each engaged forsliding in a guided manner on a cylindrical guide 44 fixed to thebracket 4. Only the cylindrical seat 43 and the corresponding guide 44disposed on the left-hand side of the axis I-I are visible in FIG. 1.Moreover, a conventional protective boot 17 is fitted between eachcylindrical seat 43 and the corresponding cylindrical guide 44.

[0032] In the embodiment described, the side 40 of the floating body 5includes two piston-cylinder units 19 of which only one is visible inFIG. 1. Each piston-cylinder unit 19 comprises a portion 45 of the side40 which defines internally a hydraulic cylinder 9 of axis a-a, havingan opening 27 facing towards the seat 12, and a piston 10 which has ahead 29 and can slide axially in the cylinder 9 so as to act on the pad14.

[0033] Ducts are present in the floating body 5, in conventional manner,for the supply of brake fluid to the cylinder 9. The cylinder 9 has anannular groove 18 for a conventional seal 28.

[0034] The piston-cylinder unit 19 also has a flexible boot 100 forprotecting the cylinder 9 and the piston 10 (particularly their slidingsurfaces) from the entry of foreign material and/or corrosive agents.

[0035] The flexible boot 100 is bellows-shaped and is such as to have arolled-up configuration comprising one or more pleats or bends when itis at rest or when the piston 10 to which it is connected is inside thecylinder 9. Moreover, the flexible boot 100 can extend to adopt asubstantially cylindrical or frusto-conical shape when the piston 10extends from the cylinder 9 towards the disk 2.

[0036] In particular, in a condition in which the pads 13 and 14 arenew, that is, when the friction linings 35, 36 are not worn (as shown byway of example in FIG. 2), the flexible boot 100 adopts the rolled-upconfiguration, whereas it is stretched out when the piston 10 extends asa result of wear of the friction linings 35, 36. FIG. 3 shows a portionof the disk brake of FIG. 1 in a condition in which the piston 9 isextended forwards as a result of the friction linings 35, 36 beingalmost completely worn out. It is pointed out that, in the view of FIG.3, the flexible boot 100 is adhering to the outer surface of the piston9 but, in the condition in which the piston 9 is extended forwards, theflexible boot 100 may also adhere to the surface substantially whilsthaving, for example, a corrugated configuration.

[0037]FIG. 4 is a partially-sectioned view of the flexible boot 100 whenit is in the rolled-up configuration.

[0038] At one end, the flexible boot 100 has a first annular region orfirst engagement ring 101 which is intended to be fixed to the portion45 defining the cylinder 9 and, at an opposite end, it has a secondannular region or second engagement ring 102 to be fixed to the piston10.

[0039] To enable the flexible boot 100 to be fixed, the portion 45 has agroove 20 for housing the first ring 101, in the vicinity of the opening27, and the piston 10 has a groove 21 for housing the second ring 102,in the vicinity of the head 29. For example, the thickness and shape ofthe engagement rings 101 and 102 are a such that they can engage in thegrooves 20 and 21 which are shaped so as to ensure the fixing of theflexible boot 100. In any case, any other suitable means may be used forfixing the flexible boot 100 to the piston-cylinder unit 19.

[0040] Moreover, an optional annular housing 22 is formed in the portion45, in the region of the opening 27, for at least partially housing theflexible boot 100 when it is in the rolled-up configuration.

[0041] The flexible boot 100 has an outer surface 103 and an innersurface 104 and comprises a plurality of concentric annular band-likeportions connected to one another and having different diameters.

[0042] In greater detail, in the rolled-up condition, the flexible boot100 comprises a first connecting band 106 between the first engagementring 101 and a second band 107 which is intended to be housed inside theseat 22 and arranged radially, that is, so as to lie in a planesubstantially perpendicular to the axis a-a of the cylinder 9 when theflexible boot 100 is fitted on the piston-cylinder unit 19.

[0043] The second band 107 is connected to a third band 108 such as todefine a cylinder of axis a-a when the flexible boot 100 is fitted onthe piston-cylinder unit 19. The third band 108 is connected to a firstpleat 109.

[0044] This first pleat 109 is connected to a second pleat 110 which isconnected, by means of a fourth band 111, to a fifth band 112 connectedto the second engagement ring 102 and arranged radially, that is, so asto lie in a plane substantially perpendicular to the axis a-a of thecylinder 9 when the flexible boot 100 is fitted on the piston-cylinderunit 19.

[0045] The first and second pleats 109 and 110 have respective annularconvex portions 113 and 114 connected to one another by an annularconcave portion 115.

[0046] It is pointed out that one or more portions of the outer surface103 can face the pad 14, that is, these portions face towards the padand, in particular, towards a wall 30 of the plate 25, when the flexibleboot 100 is fitted on the piston-cylinder unit 19. In particular, asshown in FIG. 2, the annular convex portions 113 and 114 can face thepad 14.

[0047] The flexible boot 100 advantageously has at least one spacerelement 105 which enables at least a portion of the surface 103 to bekept suitably spaced from the pad 14 and, in particular, from the wall30 of the plate 25.

[0048] In the embodiment shown in the drawings, the spacer elements 105are disposed on the annular convex portions 113 and 114.

[0049] The spacer elements 105 are advantageously intended to come intocontact with the wall 30, so as to be interposed between the outersurface 103 and the plate 25, preventing direct contact between theplate and portions of the flexible boot 100 such as, for example, theannular convex portions 113 and 114, and thus reducing the exchange ofheat between the pad 14 and the flexible boot 100 resulting from abraking operation.

[0050] According to one embodiment, the spacer elements 105 areprotuberances formed in the boot 100 and may have various shapes. Inthis connection, it should be noted that, according to one embodiment,shapes which reduce the contact area between the protuberances and thewall 30 of the plate 25 to the minimum, ideally to a point, areparticularly suitable for the purpose. In FIG. 4, the spacer elements105 are substantially spherical cap-shaped protuberances.

[0051] As further examples, FIG. 5 shows a flexible boot 100 comprisingspacer elements formed with protuberances 105′ having a substantiallyfrusto-conical shape and FIG. 6 shows a flexible boot 100 comprisingspacer elements formed with frusto-ellipsoidal protuberances 105″.

[0052] Shapes other than those shown such as, for example,frusto-pyramidal shapes or shapes of irregular cross-section (extendingfurther in one or more directions) may, however be used.

[0053] Moreover, the spacer elements 105 may be arranged on the surface103 of the flexible boot 100 with various possible surface densities.For example, the spacer elements 105 may be distributed substantiallyuniformly over at least a portion of the surface of the flexible boot(for example, the surfaces of the convex portions 113 and 114) or may bearranged so as to form groups of two or more of these elements which inturn are distributed suitably on the flexible boot 100. In thisconnection, it will be noted that, in FIGS. 4 and 5, the protuberances105 and 105′ are arranged in groups of two, whereas the protuberances105″ shown in FIG. 6 are distributed substantially uniformly.

[0054] The flexible boot 100, provided with the spacer elements 105, canbe produced in conventional manner, for example, by the injection offluidized material into a mould having cavities reproducing the boot inthe rolled-up configuration, in negative form. Typically, the materialsusable to produce the flexible boot 100 are resistant to brake fluid andcan withstand the temperatures to which the boot may be subjected.Preferably, as the temperatures which can be reached during brakingincrease, the following materials may be used, in order, for theproduction of the flexible boot 100: SBR (styrene butadiene rubber),EPDM (ethylene-propylene-diene terpolymer) or silicone rubbers, as wellas NBR (acrylomitrile butadiene rubbers) for uses in environments inwhich mineral oil is present.

[0055] As already mentioned, the flexible boot 100 is shaped in a mannersuch that, at rest, it naturally adopts the same rolled-up configurationwhich it has when it is fitted on the piston-cylinder unit 19 with newpads 13 and 14.

[0056] Moreover, in order not appreciably to change this configurationwhich is adopted naturally by the flexible boot 100, the spacer elements105 advantageously have suitably small dimensions.

[0057] For example, a spacer element 105 of the spherical-cap type mayhave a radius of between 0.3 mm and 0.9 mm or, preferably between 0.5 mmand 0.7 mm and, in particular, of about 0.6 mm.

[0058] It is pointed out that, in the stretched-out configuration, theflexible boot 100 may adopt a considerable length (dependent on thethickness of the pads used). For example, with the use of fixedcalipers, this length of the flexible boot 100 may be between 8 mm and20 mm, and with the use of floating calipers, it may be between 25 mmand 40 mm, but may even reach 50 mm.

[0059] Moreover, the flexible boot 100 typically has two or threepleats.

[0060] The flexible boot 100 preferably has openings or valves (notshown) of known type which favour the outlet of any air present betweenthe boot and the piston-cylinder unit 19.

[0061] During the mounting of the flexible boot 100 on thepiston-cylinder unit 19, which can take place in conventional manner, itis possible to arrange for the flexible boot 100 and the spacer elements105 not to be in contact with the wall 30 of the plate 25 when they arein the rolled-up configuration.

[0062] When braking takes place, the piston 10, under the pressure ofthe brake fluid, exerts a pressure on the plate 25 so that the pad 14acts on the disk 2 by means of the friction lining 35, exerting a forceon the braking band. As a result of the pressing of the pad 14 intocontact with the disk 2, an opposing force is exerted on the floatingbody 5 which is guided by the guides 44 and exerts a force in theopposite direction to the force of the piston 10.

[0063] This opposing action of the floating body 5 causes the projection42 to act on the pad 13 and hence on the disk 2, so that the brakingband of the disk is acted on by a force equal and opposite to thatexerted by the pad 14.

[0064] As stated above, during braking, the disk 2 is overheated,resulting in emission of heat which is transmitted to the pads 13 and 14and hence to the piston 10. Moreover, the heat emitted is transmitted bymeans of the piston 10 to any air trapped in the space between theflexible boot 100 and the piston-cylinder unit 19, increasing thepressure which the air exerts on the boot.

[0065] For known flexible boots, this pressure may cause an expansion ofthe flexible boot such as to bring it into contact with the pad 14 and,in particular, with the plate 25. This contact is particularly likelywhen the friction linings are new, that is, when they are not worn, sothat the flexible boot adopts the rolled-up configuration comprisingpleats extending towards the pad 14.

[0066] As already stated, the pad 14 is at a high temperature and, if itwere to come into direct contact with the flexible boot, it couldtherefore burn it or damage it.

[0067] In contrast with what may occur in conventional disk brakes, inthe disk brake of FIG. 1, the flexible boot 100 does not come intodirect contact with the pad 14, except by means of the spacer elements105. In particular, the spacer elements 105 keep the flexible boot 100spaced from the pad 14, even when the boot is expanded by the airtrapped in it.

[0068] The presence of the spacer elements 105 drastically reduces thearea of contact between the flexible boot 100 and the pad 14 and createsa space between them, preventing an exchange of heat such as to causeburning or damage to the flexible boot 100.

[0069] The presence of the spacer elements 105 advantageously preventsdirect contact of the flexible boot 100 with the pad 14, even when theboot is not arranged in the correct rolled-up configuration, as mayoccur during its mounting on the piston-cylinder unit 19 or as a resultof an operation to replace the pads 13, 14.

[0070] Moreover, it is pointed out that, by virtue of the teachings ofthe present invention, the integrity of the flexible boot can bemaintained without necessarily modifying, complicating and/or weakeningthe structure of the body which defines the cylinder in order to formsuitable channels for housing the boot.

[0071] Naturally, in order to satisfy contingent and specificrequirements, an expert in the art may apply to the above-describedflexible boot and disk brake many modifications and variations all ofwhich, however, are included within the scope of protection of theinvention as defined by the appended claims.

What is claimed is:
 1. A flexible boot for protecting a piston-cylinderunit of a disk brake including a piston slidable in a cylinder in orderto act on a brake pad, the flexible boot having a surface comprising aportion which can face a wall of the pad, characterized in that itcomprises at least one spacer element which is associated with thesurface and is intended to come into contact with the wall of the pad inorder to keep the said portion of the surface spaced from the wall ofthe pad.
 2. A flexible boot according to claim 1 in which the at leastone spacer element is a protuberance of the flexible boot which isintended to extend towards the wall of the pad.
 3. A flexible bootaccording to claim 1 in which the protuberance has the shape of aspherical cap.
 4. A flexible boot according to claim 3 in which thespherical cap has a radius of between about 0.3 mm and 0.9 mm.
 5. Aflexible boot according to claim 2 in which the protuberance isfrusto-conical.
 6. A flexible boot according to claim 2 in which theprotuberance is frusto-ellipsoidal.
 7. A flexible boot according toclaim 1, comprising a plurality of connected annular bands and at leastone pleat, so as to adopt a rolled-up configuration when the piston issubstantially inside the cylinder and a stretched-out configuration whenthe piston extends out of the cylinder.
 8. A flexible boot according toclaim 7 in which the at least one pleat has a convex portion which canface the wall, the at least one spacer element being disposed on theconvex portion.
 9. A flexible boot according to claim 1, comprising afirst annular end for engagement on the cylinder and a second annularend for engagement on the piston.
 10. A flexible boot according to claim1 made of one of the materials belonging to the group: SBR, EPDM,silicone rubber, NBR rubber.
 11. A piston-cylinder unit for a diskbrake, comprising: a portion defining a cylinder for the sliding of apiston which is to act on a brake pad, and a flexible boot, formed inaccordance with at least one of the preceding claims, for protecting thepiston-cylinder unit.
 12. A piston-cylinder unit according to claim 11in which the portion has a first seat in the cylinder, for housing thefirst annular end, and the piston has a second seat for housing thesecond annular end.
 13. A piston-cylinder unit according to claim 12 inwhich the first and second seats comprise a first annular groove and asecond annular groove, respectively, for engaging the first annular endand the second annular end, respectively.
 14. A piston-cylinder unitaccording to claim 11 in which the portion has an annular housing for atleast partially housing the flexible boot.
 15. A disk brake comprising:a brake pad for exerting a braking force on a disk associated with thedisk brake, and at least one piston-cylinder unit including a cylinderand a piston slidable in the cylinder in order to act on the brakingpad, the at least one piston-cylinder unit being formed in accordancewith at least one of claims 11 to 14.